1. Field of the Invention
The invention relates to downshifting time torque-down control device and method. More particularly, the invention relates to art for preventing engine torque from excessively decreasing due to, for example, simultaneous execution of downshifting and delay control for a knocking control measure.
2. Description of the Related Art
During downshifting of a transmission, when an input rotational speed increases and reaches a synchronous rotational speed, if a large input torque is applied to the transmission, for example, abnormal noise or shift shock may occur due to abrupt engagement of a one-way clutch related to shifting, and durability of a frictional engagement device may be impaired. In order to prevent these problems, it is suggested that engine torque be temporarily decreased before the input rotational speed reaches the synchronous rotational speed, for example, as disclosed in Japanese Patent Publication No. 5-43528.
A decrease in the engine torque may be realized by delay control of ignition timing. The delay control of ignition timing is also employed in an engine knocking control measure. For example, when normal fuel is used for an engine designed for high-octane gasoline, knocking is likely to occur. Accordingly, the ignition timing constantly moves to a delay side by a knock control system or the like. Therefore, in the case where the ignition timing moves to the delay side, when delay control is further performed in order to decrease torque during downshifting, a possibility arises where the engine torque excessively decreases and a shifting characteristic is impaired, and consequently torque shock occurs due to abrupt engagement of the one-way clutch and torque fluctuation, and acceleration performance is impaired.
For example, FIG. 9 is a time chart showing changes in a turbine speed (an input rotational speed) NT during downshifting from fourth speed to third speed, an ignition timing, and output torque. A solid line shows a case of downshifting during normal engine torque control in which the knocking control measure is not taken. When delay control is performed for delaying the ignition timing by a delay amount SBdn with respect to time T3, the turbine speed NT reaches the synchronous rotational speed smoothly (time t4), and the output torque is changed relatively smoothly. Meanwhile, a dashed line show a case in which the ignition timing is constantly on the delay side due to the knocking control measure. When the delay control is further performed by the delay amount SBdn during downshifting, the turbine speed NT does not increase due to a decrease in the engine torque. When the delay control during downshifting (hereinafter, referrer to as “downshifting time delay control”) is forcefully terminated by a backup timer or the like (time t5), shift shock occurs due to abrupt engagement of the one-way clutch, torque fluctuation and the like. Time t1 is a time at which a command for downshifting from fourth speed to third speed is output. Time t2 is a time at which actual disengagement of a frictional engagement device on a high speed side (fourth speed) is started. Time t3 is a time at which the downshifting time delay control is started. Time t4 is a time at which shifting during normal engine torque control ends. Time t5 is a time at which the downshifting time delay control is forcefully terminated by the backup timer during delay control for knocking control measure.